Default secretory pathway and exocytosis
A protein in the endoplasmic reticulum with no extra sorting signal (address label) travels in vesicles to the plasma membrane, where exocytosis releases or inserts it.
Remember
A protein that has entered the endoplasmic reticulum ends up at the cell surface if no extra signal diverts it: vesicles carry it there, and exocytosis releases it or leaves it in the plasma membrane.
Memory aid
Think of the endoplasmic reticulum as a sorting office with a default van: anything not stamped with a different destination goes to the cell surface.
Ask whether the protein has an unusual destination, and if it does not, assume the vesicle route to the surface.
Proteins bound for the cell surface or the outside of the cell are made by ribosomes on the rough endoplasmic reticulum (ER, the membrane network where proteins are folded). They enter it through an initial entry signal (the signal that takes a protein into the ER) and then go through the Golgi apparatus. The route from the ER through the Golgi apparatus to the cell surface is the secretory pathway.
A protein in this pathway with no further sorting signal (an address label) follows the default route. It is packed into transport vesicles (small membrane-bound bubbles), which travel to the cell surface and fuse with the plasma membrane. The fusion of a transport vesicle with the plasma membrane is called exocytosis.
Secreted proteins are then released outside the cell. Membrane proteins stay in the plasma membrane, because the vesicle membrane merges with the cell membrane and its proteins become part of it.
An extra signal diverts a protein from this route. KDEL (a retrieval sequence) returns a protein to the ER, and mannose-6-phosphate (a tag) sends an enzyme to the lysosome. Without one, the vesicle route to the surface wins.
Some specialised cells store vesicles and release them only after a release signal (a different kind of signal from a sorting signal), such as a rise in calcium. This is called regulated secretion, and the underlying step of vesicle fusion is the same. For protein secretion, think vesicles, not channels and not diffusion.
Key facts
- Default route
- a protein in the secretory pathway with no extra sorting signal travels in transport vesicles to the cell surface.
- Process
- vesicles fuse with the plasma membrane in exocytosis.
- Outcomes
- on the default route, a secreted protein is released outside the cell, and a membrane protein stays in the plasma membrane.
- Exceptions
- KDEL retrieves a protein to the endoplasmic reticulum, and mannose-6-phosphate sends it to the lysosome.
- Entry
- ribosomes on the rough endoplasmic reticulum make the proteins that are secreted.
- Regulated secretion
- specialised cells store vesicles and release them only after a release signal, such as a rise in calcium.
Exam traps
- Vesicles that mainly carry material backwards from the Golgi apparatus to the endoplasmic reticulum (COPI vesicles) do not carry secreted proteins onwards to the plasma membrane.
- Vesicles that carry proteins from the endoplasmic reticulum to the Golgi apparatus (COPII vesicles) do not fuse directly with the plasma membrane.
- A mannose-6-phosphate tag does not carry a protein to the cell surface, because it sorts enzymes to lysosomes.
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